This invention relates to a pipe coupling used with an automatic connecting and disconnecting device for hydraulic and pneumatic pipings for general industrial machinery, such as disclosed in Japanese patent publication gazette No. S51-41934.
U.S. Pat. No. 4,903,730 discloses a pipe coupling of this type as described below with reference to FIG. 1 of the accompanying drawings. This coupling comprises a first main body 2 fixed to a stationary face plate 1 and a second main body 4 fixed to a movable face plate 3. The first main body 2 is cylindrical in shape and has a joint mouth 5 in its front end face and a pipe joint at its rear end. The second main body 4 has a front end portion having an outer diameter fitting in the joint mouth 5 and includes a second valve hole 8 in its front end face 7 and a pipe joint 9 at its rear end. The first and second main bodies 2 and 4 have a common axis when they are fixed to the respective face plates 1 and 3 and the second main body 4 is adapted to move close to and apart from the first main body 2 along the axis with movement of the movable face plate 3. The first main body 2 encloses therein a valve seat 10 freely sliding along the same axis.
The valve seat 10 is urged by a first spring 12 toward the joint mouth 5 and has an annular projection at its rear end which butts against an annular step 13 of the first main body 2. The valve seat 10 has a circular hole 35 formed in its front end face and a tubular third check valve 31 is slidably and fluid-tightly fit in this hole 35. The inner hole of the third check valve 31 constitutes a first valve hole 11 and a valve seat 36 is formed near its front end and adapted to be closed by a first check valve 14 which is fixedly supported by a central connecting rod 18 fixed to the first main body 2. The third check valve 31 is urged forward by a spring 40 disposed between an annular projection 37 formed on its outer surface and a spring seat 39 fit in an inner hole 38 of the valve seat 10 to close a gap between the valve seat 10 urged by the first spring 12 into its advance position and the first check valve 14 in its original position. Numeral 41 denotes a holding ring fitting in the inner hole 38 of the valve seat 10 and butting against the spring seat 39. It has through holes 42 and a central hole in which the connecting rod 18 fits slidably.
The second main body 4 includes therein a second check valve 20 of a greater diameter than the first check valve 14, which is adapted to be pushed by a second spring 19 to close the second valve hole 8 from its inside. An annular sealing member 21 is disposed on the front face of the valve seat 10 to face the front end face of the second main body 4. The sealing member 21 is adapted to enclose the first and second valve holes 11 and 8 when the front end face 7 of the second main body 4 butts against the front face of the valve seat 10 into a connected state of the coupling.
The reason why the third check valve 31 is disposed between the valve seat 10 and the first check valve 14 in the abovementioned structure of the first main body 2 is that it is practically impossible to put the valve seat 10 in contact with both the annular step 13 and the first check valve 14 at the same time and, if only the valve seat 10 contacts the first check valve 14, this check valve structure may be broken by a high fluid pressure acting to the valve seat 10, while the fluid pressure acting on the third check valve 31 is much lower due to its small cross sectional area.
However, the abovementioned prior art pipe coupling has such a disadvantage in that its size is unnecessarily large due to addition of the third check valve 31 when the coupling is used at a relatively low flow rate, since it is preferable to reduce the outer diameter of the coupling to make its overall size as small as possible.